Method of fabricating optical fiber or optical device doped with reduced metal ion and/or rare earth ion

ABSTRACT

Disclosed is a method of fabricating an optical fiber or an optical device doped with reduced metal ion and/or rare earth ion, comprising steps of: forming a partially-sintered fine structure in a base material for fabricating the optical fiber or the optical device; soaking the fine structure into a doping solution containing a reducing agent together with metal ion and rare earth ion during a selected time; drying the fine structure in which the metal ion and/or rare ion are/is soaked; and heating the fine structure such that the fine structure is sintered.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a technology for fabricatingan optical fiber or an optical device, and more particularly to a methodof fabricating an optical fiber or an optical device doped with reducedmetal ion(s) and/or rare earth ion(s).

2. Description of the Related Art

An optical fiber containing metal ion and/or rare earth ion is broughtunder a special optical fiber, since it can be variously applied to anoptical amplifier or an optical switching device etc. Therefore, much ofthe research in this area has been performed.

One of the research projects is a technique of reducing doped metal ionand/or rare earth ion, Generally, an atom has a different energy leveldistribution depending on its valence, and therefore has differentspectroscopic characteristics such as light absorption and lightemission. Accordingly, a little more diverse light absorption and lightemission can be obtained by utilizing the change of valence and therebythe optical fiber and the optical device having various opticalamplification and optical switching characteristics can be obtained.

As an example, let us consider rare earth ions, when a rare earth ionhas the valence of 3+, the light absorption characteristic due toelectronic transition between 4ƒ electron orbit and 5d electron orbitoccurs only in an ultraviolet wavelength region, whereas when thevalence of the rare earth ion changes to 2+ ion, such an lightabsorption characteristic occurs in both visible and infrared wavelengthregions. For this reason, a technique of making doped metal ion and rareearth ion with desired valences, respectively is required. Furthermore,every atom has its own valence states in which the atom is mainlyexisted in nature and thus a specific process is required in order totransfer the valence into another valences.

For example, most of the rare earth ions have the valence of 3+. Inorder to stably transfer the valence of 3+ into the valence of 2+, 1+ or0, it is necessary to reduce the rare earth ions. There have beenproposed various reduction treatment methods as described below.

Firstly, there is a method of applying gamma rays to the rare earthmetal ion having the valence of 3+. For example, it is reported thatTm²⁺ can be obtained, if the gamma rays is applied to a CaF₂ crystalcontaining Tm³⁺.

However, in this method, there is a problem that a gamma ray source isdangerous to handle and the cost required in handling it safely is thusexpensive.

Secondly, there is another method in which an aerosol type material isutilized. In this method, a MCVD (modified chemical vapor deposition)process is indispensable. In other words, this method includes the MCVDprocess in which a glass layer containing rare earth ions is depositedin a quartz glass tube, using material having aerosol formulation whichgenerates carbon, together with a powder which generates rare earth ionand glass when fired. Then, processes of removing the carbon and OHradical, sintering the glass and collapsing the glass tube are, in turn,performed to thus obtain an optical fiber preform. For example, in aglass optical fiber having SiO₂—Al₂O₃ components, Eu²⁺ and SM²⁺ arereduced from Eu³⁺ and Sm³⁺, respectively.

To date, this method which utilizes the material having aerosolformulation is performed through only the MCVD process. A desired rareearth ion material having aerosol formulation and an additionalapparatus for supplying material having aerosol formulation are needed.

Further, there is a method of injecting a mixture of H₂ and Ar gases andobtaining the reduced rare earth ion during melting of glass. Forexample, in a glass having SiO₂—Al₂O₃ components or SiO₂—B₂O₃components, Sm²⁺ is reduced from Sm³⁺.

In this method, there is a problem that processes of fabricating theoptical fiber preform are complicated in comparison with theconventional processes and are not yet commercialized.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a methodof fabricating an optical fiber or an optical device, in which metal ionand/or rare earth ion safely and facilely reduced, in comparison withthe prior art methods, together with the utilization of the prior artprocesses of fabricating the optical fiber and/or the optical device.

To achieve the aforementioned object of the present invention, a methodaccording to the present invention is characterized by forming apartially-sintered fine structure in a base material for fabricating anoptical fiber or an optical device and soaking the fine structure into adoping solution containing a reducing agent together with metal ionand/or rare earth ion during a selected time, thus doping the finestructure with the metal ion and/or rare earth ion together with thereducing agent. Therefore, reduced metal ion and/or rare earth ionthrough the reducing agent is obtained.

One method according to the present invention of fabricating an opticalfiber or an optical device doped with reduced metal ion and/or rareearth ion comprising the steps of: forming a partially-sintered finestructure in a base material for fabricating the optical fiber or theoptical device; soaking the fine structure into a doping solutioncontaining a reducing agent together with metal ion and rare earth ionduring a selected time; drying the fine structure in which the metal ionand rare ion is soaked; and heating the fine structure such that thefine structure is sintered.

Another method of fabricating an optical fiber or an optical devicedoped with reduced metal particle and/or rare earth element, comprisingsteps of: forming a partially-sintered fine structure in a base materialfor fabricating the optical fiber or the optical device; soaking thefine structure into a doping solution containing a reducing agent havingstrong reduction potential together with metal ion and rare earth ionduring a selected time; drying the fine structure in which the metal ionand/or the rare earth ion is/are soaked; and heating the fine structuresuch that the fine structure is sintered, thereby forming the metalparticle and/or the rare earth elements.

Preferably, the reducing agent is hydrocarbon compounds. Glucose,sucrose, glycerine, dextrin, benzene, phenol, hexane, toluene, stylene,naphthalene, and the like are exemplified.

In addition, the reducing agent is alkoxide compounds. TEOS (tetraethylorthosilicate), TMOS (tetramethyl orthosilicate), TEOC (tetraethylorthocarbonate), TMOC (tetamethyl orthocarbonate) and the like areexemplified.

Preferably, the metal ion and/or rare earth ion is at least one ionselected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th,Dy, Ho, Br, Tm, Yb, Lu, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y,Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Hf, Ta, W, Re, Os, Ir, Pt,Au, Tl, Pb, Bi and a mixture thereof.

Further, the base material for fabricating the optical fiber or theoptical device has a basic composition comprising a silicon oxide or acomposite oxide of a silicone oxide and an oxide; in which the oxide isat least one selected from the group consisting of germanium oxide(GeO₂), boron oxide (B₂O₃), phosphorous oxide P₂O₅), and titanium oxide(TiO₂).

Preferably, the base material for fabricating the optical fiber or theoptical device has a basic composition selected from silica (SiO₂),germa osilicate (SiO₂—GeO₂), phosphorosilicate (SiO₂—P₂O₅),phosphorogermanosilicate (SiO₂—GeO₂—P₂O₅), borosilicate (SiO₂—B₂O₃),borophosphorosilicate (SiO₂—P₂O₅—B₂O₃), borogermanosilicate(SiO₂—GeO₂—B₂O₃), titanosilicate (SiO₂—TiO₂), phosphorotitanosilicate(SiO₂—TiO₂—P₂O₅), or borotitanosilicate (SiO₂—TiO₂—B₂O₃).

Preferably, the step of forming the partially-sintered fine structure inthe base material for fabricating the optical fiber or the opticaldevice is performed by a process selected from MCVD (modified chemicalvapor deposition), VAD (vapor-phase axial deposition), VOD (outsidevapor deposition), FHD (flame hydrolysis deposition), etc.

The optical device in the present invention includes a planar opticalamplifier, an optical communication laser, and a planar optical switchdevice, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and another advantages of the present invention willbecome more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a schematic view showing an apparatus for performing processesof the present invention;

FIG. 2 is graph showing a light absorption spectrum of an optical fiberdoped with reduced rare earth ion(Tm⁺²) fabricated by a first embodimentof the present invention;

FIG. 3 is a graph showing a light absorption spectrum of an opticalfiber fabricated by a comparative example 1 without using the reducingagent;

FIG. 4 is a graph showing a light absorption spectrum of an opticalfiber doped with reduced rare earth ion (Eu⁺²) fabricated by a secondembodiment of the present invention;

FIG. 5 is a graph showing a light absorption spectrum of an opticalfiber fabricated by a comparative example 2 without using the reducingagent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, preferred embodiments of the present invention will be described indetail with reference to the annexed drawings.

A method of the present invention comprises a step of forming apartially-sintered fine structure in a base material for fabricating anoptical fiber or an optical device, and a step of soaking the finestructure in a doping solution containing a hydrocarbon compound as areducing agent together with metal ion and/or rare earth ion for 1 to1.5 hours. That is, the fine structure of the base material is dopedwith the reducing agent together with metal ion and/or rare earth ionand the metal ion and/or rare earth ion reduced by the doped reducingagent is thereby obtained.

The method of the present invention is a modification of a solutiondoping technique for adding the rare earth ion and/or metal ion to theoptical fiber or the optical device. This solution doping technique is amethod of doping the metal ion or the rare earth ion in an optical fibercore, which can be utilized with any of conventional methods offabricating an optical fiber preform, such as MCVD (modified chemicalvapor deposition), VAD (vapor-phase axial deposition), OVD (outsidevapor deposition), etc. The solution doping technique is also used asthe technique which may dope all of the rare earth ion and/or the metalion, capable of being formed into a solution type, even in a method offabricating a plane glass optical device through a FHD (flame hydrolysisdeposition) process.

For example, the solution doping technique please refer to J. E.Townsend, et al. “Solution for fabrication of rare earth doped opticalfibers”, Electron, Lett., Vol. 23, p.p. 329-331, 1987) through the MCVDprocess is as follows. Herein, in order to obtain reduced rare earthion, an aqueous solution in which sucrose as a strong reducing agent isdissolved together with rare earth chloride is used as a dopingsolution. First, a core layer partially sintered and then having aplurality of pores is formed in a silica tube using the conventionalMCVD process (please refer to MacChesney et. al., “Optical fiberfabrication and resulting product”, U.S. Patent, 1997). Then, the silicatube is filled with the aqueous solution in which the sucrose isdissolved together with the rare earth chloride. The aqueous solution ishold for 1 to 1.5 hours in order for the solution to be sufficientlypermeated into the pores of the core layer and is then discharged. As aresult, the doping solution remains in the pores. The core layer dopedwith the aqueous solution is dried while the silica tube is hold at atemperature of 100 to 250° C. with the passage of an inert gas such ashelium gas only, using the MCVD process. At this time, ethanol andmoisture is removed. Sequentially, using hydrogen-oxygen flames, thecore layer is heated at a high temperature of 2000° C. until carbongenerated from the sucrose is removed and the core layer is thencompletely sintered (referring to M. F. Yan, et al., “Sintering ofoptical wave-guide glasses”, J. of Mater. Sci., p.p. 1371-1378, 1980).After that, the optical fiber preform is fabricated through a collapsingstep in which the tube is heated to more than 2200° C. with thecontinuous purging of the inert gas, using the hydrogen-oxygen flames.The optical fiber preform is drawn to produce the optical fiber dopedwith the reduced rare earth ion.

The sucrose contained in the doping solution is composed of C, H and Ocomponents. During the above drying step, most of the H and O componentsamong the above components are removed and only the carbon (C) isremained. The carbon (C) is combined with Oxygen (O₂) remained at thehigh temperature of about 2000° C. to form carbon monoxide (CO), andthus carbon monoxide reduces the doped rare earth ion. At this time, thereaction temperature at which the carbon monoxide (CO) is formed isdecided within the possible range of reduction of the rare earth ion,using an Ellingham Diagram. At the same time, a strong reductionatmosphere is created by injecting only the inert gas into the silicaglass tube, so that the carbon (C) can be fully participated in thereduction reaction of rare earth ion. Further, preferably, the inert gasonly is also passed through during the collapsing step for fabricatingthe optical fiber preform, thereby creating a reduction atmosphere atits maximum.

Embodiment 1

First, thulium chloride hexahydrate (TmCl₃.6H₂O) of 0.04M and sucrose(C₁₂H₂₂O₁₁) of 2.17M are dissolved in deionized water to prepare adoping solution containing rare earth ion (Tm³⁺) and the sucrose as areducing agent. Herein, a hydrocarbon compound or an alkoxide compoundis used as the reducing agent.

As shown in FIG. 1, a porous fine structure is formed through an MCVDprocess at an inner wall of a silica glass tube having an inner diameterof 19 mm and an outer diameter of 25 mm so that the portion thereof toform an optical fiber core has a basic glass composition of SiO₂—GeO₂.The doping solution fabricated is injected into the above glass tube andthen discharged after 1 hour. Then, the core layer is dried by heatingthe glass tube again at a temperature of 100 to 250° C. using the MCVDapparatus with the purge of only helium gas through the glass tube.

Then, the above sintering step and collapsing step are repeatedlyperformed 8 times and 15 times, respectively, at a temperature of 2000°C., thereby obtaining the optical fiber preform doped with Tm²⁺ ion. Theoptical fiber preform is drawn to fabricate the optical fiber. Herein,even when a sintering step is carried out at a temperature of 1600 to2200° C., the same result is also obtained.

A light absorption spectrum of the optical fiber fabricated using thedoping solution containing the sucrose as a reducing agent is shown inFIG. 2. Light absorption spectrum of FIG. 2 shows that light absorptionspectrum shown at 465 nm, 680 nm, 785 nm, 1210 nm, and 1600 nm is formedby Tm³⁺ ion, and light absorption spectrum distributed in the broadrange of 400 nm and 900 nm is formed by Tm²⁺ ion.

COMPARATIVE EXAMPLE 1

Thulium chloride hexahydrate (TmCl₃.6H₂O) of 0.04M and aluminum chloridehexahydrate (AlCl₃.6H₂O) of 0.19M are dissolved in ethanol to prepare adoping solution without containing the sucrose as the reducing agent.

A core layer having a porous fine structure is formed at an inner wallof a silica glass tube in the same method as the embodiment 1. Thefabricated doping solution is injected into the glass tube and thendischarged after 1 hour. Then, the core layer is dried together with thepurge of helium, oxygen and chlorine through the tube.

Then, the above sintering step and collapsing step are repeatedlyperformed 3 times and 7 times, respectively, at a temperature of 2000°C., thereby obtaining the optical fiber preform doped with Tm³⁺ ion. Theoptical fiber preform is drawn to fabricate the optical fiber.

A light absorption spectrum of the optical fiber fabricated by using thedoping solution without containing the sucrose as the reducing agent isshown in FIG. 3. Differently from the result of the embodiment 1 usingthe reducing agent, FIG. 3 shows only light absorption spectrumaccording to Tm₃₊ ion.

Embodiment 2

Europium chloride (EuCl₃.xH₂O) of 0.097M and sucrose (C₁₂H₂₂O₁₁) of0.518M are dissolved in deionized water to prepare a doping solutioncontaining rare earth ion (Eu³⁺) and the sucrose as a reducing agent.

Then, the optical fiber doped with Eu²⁺ ion is fabricated by the sameprocesses as in the embodiment 1.

A light absorption spectrum of the optical fiber fabricated by using thedoping solution containing the sucrose as the reducing agent is shown inFIG. 4. In the light absorption spectrum of FIG. 4, the light absorptionspectrum distributed in the broad range of 600 nm and 1200 nm is formedby Eu₂₊ ion. This spectrum is not shown in the case of Eu³⁺ ion.

COMPARATIVE EXAMPLE 2

Europium chloride hydrate (EuCl₃.xH₂O) of 0.097M and aluminum chloridehexahyrate (AlCl₃ 6H₂O) of 0.518M are dissolved in ethanol to prepare adoping solution without containing the sucrose as the reducing agent.

Then, the optical fiber doped with Eu³⁺ ion is fabricated by the sameprocesses as in the comparative 1.

A light absorption spectrum of the optical fiber fabricated by using thedoping solution without containing the sucrose as the reducing agent isshown in FIG. 5. Differently from the result of the embodiment 2 usingthe reducing agent, FIG. 5 shows only light absorption spectrumaccording to Eu³⁺ ion.

The above embodiments 1 and 2 illustrate that the optical fibers performdoped with Tm²⁺ ion and Eu⁺² ion, respectively, by the doping solutionscontaining the reducing agents are obtained. Depending on the intensityof reduction potential which the reducing agent has, it is confirmedthat metal ion or rare earth ion having 3+ valence is changed to 2+valence or 1+, in some cases to “0” valence. When the metal ion or rarethe earth ion is reduced to “0” valence, an optical fiber preform or anoptical device preform doped with metal particle or rare earth elementis formed.

As described above, the present invention can fabricate an opticaldevice doped with reduced metal ion and/or rare earth ion having adesire valence by a facile solution doping technique.

According to the present invention, an optical fiber or an opticaldevice doped with metal ion and/or rare earth ion reduced by the facilesolution doping technique, with no need to change the conventional MCVD,VAD, OVD processes, etc. may be fabricated.

While the present invention has been described in detail, it should beunderstood that various changes, substitutions and alterations can bemade hereto without departing from the spirit and scope of the inventionas defined by the appended claims.

1. A method of fabricating an optical fiber or an optical device dopedwith reduced metal ion and/or rare earth ion, comprising steps of:forming a partially-sintered fine structure in a base material forfabricating the optical fiber or the optical device; soaking the finestructure into a doping solution containing a reducing agent togetherwith metal ion and rare earth ion during a selected time; drying thefine structure in which the metal ion and/or rare ion are/is soaked; andheating the fine structure such that the fine structure is sintered. 2.The method of claim 1, wherein the reducing agent is hydrocarboncompounds.
 3. The method of claim 2, wherein the hydrocarbon compound isany one selected from the group consisting of glucose, sucrose,glycerine, dextrin, benzene, phenol, hexane, toluene, stylene, andnaphthalene.
 4. The method of claim 1, where the reducing agent isalkoxide compounds.
 5. The method of claim 4, wherein the alkoxidecompound is any one selected from the group consisting of TEOS(tetraethyl orthosilicate), TMOS (tetramethyl orthosilicate), TEOC(tetraethyl orthocarbonate), and TMOC (tetramethyl orthocarbonate). 6.The method of claim 1, wherein the metal ion and/or rare earth ion is atleast one selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Sc, Ti, V, Cr, Mu, Fe, Co, Ni, Cu,Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Hf, Ta, W, Re, Os,Ir, Pt, Au, Tl, Pb, Bi and a mixture thereof.
 7. The method of claim 1,wherein a base material for fabricating the optical fiber or the opticaldevice has a basic composition comprising a silicon oxide or a compositeoxide of a silicone oxide and an oxide; in which the oxide is at leastone selected from the group consisting of germanium oxide (GeO₂), boronoxide (B₂O₃), phosphorous oxide (P₂O₅), and titanium oxide (TiO₂). 8.The method of claim 1, wherein a base material for fabricating theoptical fiber or the optical device has a basic composition selectedfrom silica (SiO₂), germanosilicate (SiO₂—GeO₂), phosphorosilicate(SiO₂—P₂O₅), phosphoroger anosilicate (SiO₂—GeO₂—P₂O₅), borosilicate(SiO₂—B₂O₃), borophosphorosilicate (SiO₂—P₂O—B₂O₃), borogermanosilicate(SiO₂—GeO₂—B₂O₃), titanosilicate (SiO₂—TiO₂), phosphorotitanosilicate(SiO₂—TiO₂—P₂O₅), or borotitanosilicate (SiO₂—TiO₂—B₂O₃).
 9. The methodof claim 1, wherein, the step of forming the partially-sintered finestructure in the base material is performed by a process selected fromthe group consisting of MCVD (modified chemical vapor deposition), VAD(vapor-phase axial deposition), VOD (outside vapor deposition), and FHD(flame hydrolysis deposition).
 10. A method of fabricating an opticalfiber or an optical device doped with reduced metal particle and/or rareearth element, comprising steps of: forming a partially-sintered finestructure in a base material for fabricating the optical fiber or theoptical device; soaking the fine structure into a doping solutioncontaining a reducing agent having strong reduction potential togetherwith metal ion and rare earth ion during a selected time, drying thefine structure in which the metal ion and/or the rare earth ion is/aresoaked; and heating the fine structure such that the fine structure issintered, thereby forming the metal particle and/or the rare earthelements.
 11. The method of claim 10, wherein the reducing agent ishydrocarbon compounds.
 12. The method of claim 10, where the reducingagent is alkoxide compounds.